Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: Danskman on September 09, 2009, 07:48:43 am
-
Hello!
Got one these on my bench these days, and don't remember how to set hte DC balance and Bias Adjust pots on the amp circuit.
I worked on SS amps about 30 years ago and let them for computers. No further doc an this subject can be found around for this particuiar amp, apart the schem and the layout.
Is there a tech guru around who could explain some theory and the proper way to set them? I adjusted the pots trying to limit hum and the nasty distortion that appears when the signal level is low, but I hate to work this way...
(I joined the power amp schema in a zipped pdf)
Many thanks in advance and best regards,
Danskman
-
Bias Adj is set to give 0.050V (50mV) across the output emitter resistors (here 0.27 ohms) with no load nor signal. The danger is that slight mis-adjustment of Bias Adj will give WAY too much current and melt the outputs. I would start with Q15 Q16 disconnected, adjust to find where voltage on Q12's 4.7 ohm resistor goes form zero to 0.050V or 0.1V, set back to nearly-zero. Now when Q15 and Q16 are installed, the 0.27 ohms will show about zero. The amp will work this way, but "hoarse" on small signals. Carefully sneak Bias Adj for the 0.05V on the 0.27 resistors.
DC Bal is simply set for zero V DC output, no signal no load. This type circuit should not need this trim, but they munged it. Or were using old-world thinking on this balanced-pair design. On a guitar amp, I would not fret about 0.1V residual DC on the output, It sure won't affect a guitar speaker.
This amp has -NO- protection against shorts. Anything bad happens, failures cascade from output into the Q10 Q11 stage.
It does have dual feedback. The output impedance is not dead-zero like many SS amps. In fact it may be as high as 50 ohms, "no damping" by hi-fi standards, and very slight damping by guitar-speaker standards.
Neither output jack may be grounded. Which means 1/4" plugs are a bad idea (the exposed shell is prone to be grounded). Of course there's no alternative to 1/4" on stage-amps.
-
Thanks, PRR!
I will get the new pots today , as the original ones are defective...) and will apply the mothodology you described to set this amp up.
I definitely prefer to work on tubes amps... These SSs often use odds and hard-or-nearly-impossible-to-find transistors! :wink:
Thanks again for the lesson and the precious time you spent for me!
With respect,
Danskman
-
Repair is done. Following your procedure, the amp is working now. This is really a poorly designed circuit, IMHO (and apparently IYO :wink:)... but it works. The future user will have to be carefull with it, but it's a 2x12 combo and he/she should not play with the loudspeaker connection. I placed a warning/ information note on back of the chassis, just in case.
Once again, THX for your help and best regards,
Danksman
PS: it's a TR-75